This specification relates generally to thermal comfort and more particularly to a system for providing pressurized, thermally conditioned air to a convective device under manual remote control. Another aspect concerns the control of a forced air warming unit by regulation of both the flow rate and the temperature of the air exiting the forced air warming unit in response to a single manual act or manual operation of a single element of control.
It is known to use therapeutic warming to treat patients perioperatively for hypothermia in order to mitigate the risk of adverse outcomes such as increased rates of wound infection, lengthened hospital stays, and increased mortality rates. Hypothermia occurs when the core body temperature falls below 36° C.; mild hypothermia occurs when core body temperature is in the range of 34° C. to 36° C. The clinical effectiveness of therapeutic warming depends upon delivery of enough heat to a patient to raise the patient's core body temperature to, or maintain it within, a narrow range, typically near 37° C. This range is called “normothermic” and a body with a core temperature in this range is at “normothermia”.
Therapeutic warming is contrasted with “comfort warming” which is intended to maintain or enhance a patient's sense of “thermal comfort”. Of course, therapeutic warming may also comfort a patient by alleviating shivering, for example, but this is a secondary or ancillary effect. Thermal comfort is a subjective notion; however, the environmental conditions necessary to produce a sense of thermal comfort in a population of human beings are known and well tabulated. See, for example, P. O. Fanger, THERMAL COMFORT: Analysis and Applications in Environmental Engineering, Danish Technical Press, 1970, pp. 5-67. Fanger defines thermal comfort as “that condition of mind which expresses satisfaction with the thermal environment.” Even when a patient is normothermic, less than ideal environmental conditions can result in acute feelings of thermal discomfort. Under normothermic conditions, thermal comfort is largely determined with reference to skin temperature, not core body temperature.
Aside from humanitarian concerns, there are many good reasons to provide for the thermal comfort of a patient in a clinical setting. For example, attending to the thermal comfort of patients will reduce the time nurses spend responding to patients' requests for thermal comfort interventions. Lack of thermal comfort is a frequent complaint among patients, and results in poor patient ratings for clinics and hospitals. Ensuring the thermal comfort of patients will enhance patient satisfaction. Furthermore, the provision of means to enable a patient to selectively control the thermal characteristics of his or her own personal microenvironment in a clinical setting should produce the following additional unexpected benefits: 1.) reduced blood pressure providing easier entry to blood vessels for IV access; 2.) reduced pain sensation; 3.) normalizing of the patient's perception of time slowing; 4.) reduced anxiety and reduced need for medication. These and other objectives are realized when a patient is maintained in a state of thermal comfort. With comfort warming, it is also possible to slightly raise the core body temperature to store enough heat so that normal body heat loss during the course of a short operation will not lead to hypothermia.
Patient warming in a clinical environment may be provided by convective devices that receive and distribute warmed, pressurized air, inflate, and then expel the distributed air through one or more surfaces toward a patient. Examples of convective devices that deliver therapeutic warming are thermal blankets that are deployed over prone patients. See, for example the thermal blanket described in commonly-owned U.S. Pat. No. 7,090,692. Comfort warming may be provided by convective means such as the convectively operated warming devices described in commonly-owned international Publication No. WO 2003/086500, incorporated herein by reference. In fact, such warming devices may be constructed to be operated in a clinical mode, under control of health care personnel, to deliver therapeutic warming, and in a comfort mode, which may be controlled by the patient, to deliver comfort warming; see commonly-owned US Publication No. U.S. 2006/012267, also incorporated herein by reference.
The temperature requirements for thermal comfort are variable. They depend on the environment, and also on personal and subjective factors. As a result, people usually desire to have direct control over their own thermal environment. A convectively operated warming device according to the incorporated publications includes a clinical garment such as a hospital gown that forms a thermal microenvironment in the space between the garment and a patient's body. The warming device further includes an inflatable convective apparatus on an inside surface of the clinical garment that vents warmed air into the thermal microenvironment. In the incorporated U.S. Pat. Nos. 7,014,431 and 6,876,884, a forced-air warming unit is described which provides a warmed pressurized air stream for operation of the convective apparatus. The forced air warming unit includes a manually-operated remote control that enables a patient to vary air stream temperature.
A thermal microenvironment should suit an individual's notion of thermal comfort to the fullest extent possible. The ability to vary air temperature alone affords only a gross control over the thermal microenvironment. In order to meet a patient's subjective standard of thermal comfort, it is desirable for the patient to have control over more than one factor contributing to the thermal microenvironment. In this regard, control of the air flow rate and control over the temperature of an air stream would enable more sensitive adjustment of the thermal characteristics of the microenvironment. It would be possible to adapt forced air warming unit designs for finer, more sensitive control by provision of separate blower speed and temperature controls, but dual-control operation may be confusing to the patient and may lead to more anxiety and discomfort through a perceived loss of effect because of unpredictable results. In the manual control apparatus and method to be described, an individual is afforded a simple, unambiguous way to control both air flow rate and air temperature in a stream of air emitted by a forced air warming unit by means of a single manually-operated control element.